The present invention relates to optical fiber communication, and more particularly to optical fiber communication for local area network (LAN) applications using frequency-division-multiplexing.
Optical fiber systems have not succeeded in providing multi-channel digital, audio, broadband video and facsimile transmission in an integrated LAN network. The present invention provides an integrated LAN network using optical fibers.
The transmission of video signals using digital techniques requires a very large bandwidth. Conventional fiber optic systems have bandwidth limitations due to non-linear distortion and therefore can readily be saturated for such applications. Further, conventional coaxial cable systems do not have the required bandwidth. Existing optical fiber LANs use time-division-multiplexing (TDM) to achieve multi-channel digital operation. Such systems lack the flexibility in transmitting analog and digital information simultaneously, particularly for real time broadband analog signals.
Conventional frequency-division-multiplexing (FDM) in optical fiber communication (OFC) utilizes a semi-coherent optical source which is directly intensity modulated by a multiplexed RF signal. Direct detection is employed to recover the RF signals. These RF signals up to several hundred megahertz are demodulated by standard RF techniques. This system has been widely used for multiple-channel video transmission normally over a distance of up to 10 kilometers. Because of serious intermodulations, such a system is normally implemented using FM modulated RF channels. This increases the complexity of the system and therefore the cost, and reduces the reliability of the system.
It is recognized for coherent optical fiber communication (OFC) that it is possible to use optical modulation techniques which allow either one of the sidebands or carrier to be suppressed to provide dispersion compensation. Such techniques include Time-Domain-Holography Modulation (TDHM) for analog transmission and Double-Beam Modulation (DBM) for digital transmission, see the article entitled, "Feasibility and Requirements For Dispersion Compensation in Coherent FOC," by C. S. Ih in Optical Waveguide Sciences-Proceedings of the International Symposium, held at Kweilin, People's Republic of China (PRC), June 20-23, 1983, Martinus Nijhoff Publishers, 1983.
Also U.S. Pat. No. 4,210,803, in the name of Charles C. Ih, issued July 1, 1980, discloses the use in optical fiber communication (OFC) of two or more substantially monochromatic optical beams having their frequencies interlocked to a preselected degree dependent upon the transmission service to be effected, one beam being reserved as the carrier or reference while the other beams are reserved as information beams upon which the individual electrical signals to be transmitted are modulated, and the implementation of this technique using integrated optics technology.